The present invention relates to shaped work pieces and parts that are subjected to wear in actual use and that are designed with a wear resistant coating made of a hard substance. In particular, the invention relates to providing improved work parts which are used in an environment where they are subjected to wear and abrasion because the work part performs as a sealing surface that slides along a countersurface, or surface with which the part comes into contact. A hard substance protective coating that is wear resistant is formed on the surface of the work part by a brazing technique.
In a further aspect the invention also relates to the process of producing the wear resistant coatings on the work piece.
Designing parts that are subject to wear with protective coatings made of hard substances is a generally known technology and various procedures are known in the art for applying the hard coatings.
Year after year considerable economic losses are caused by the effects of various forms of wear on technical structures. Economic and technical considerations as well as increased awareness of the limited availability of valuable raw materials are the factors underlying the rising importance of wear protection technology in recent years. This development has been spurred on by mounting demands regarding the reliability of functional surfaces under often extreme stresses of thermal, electrochemical and mechanical nature. Efforts to improve performance data and efficiency in the machine industry, plant construction and automobile manufacturing sectors, for example, have led to the imposition of more stringent demands on the materials being used. This, in turn, has brought about the functional separation of tribologically stressed surfaces and the auxiliary components of a work piece which are, in most cases, exposed to far less stress. Surfaces can therefore be ideally matched to the stresses they will encounter in actual use. This functional separation is achieved by the deposition of suitable protective surface layers, this definition covering the whole of the extensive field of coating technology.
At present, there are a large number of procedures available for the deposition of such coatings. The scope extends from chemical and electrochemical procedures to those for the deposition of ultra-thin layers by vapor phase from thermal spraying procedures to those for plating and surfacing, by which layers in the millimeter range up to several centimeters in thickness can be obtained.
The choice of procedure for a particular application is determined from a technical point of view by the load cycle acting on the composite material, and its required specification. Further factors which are decisive in the selection of one procedure or another are the economic aspects of the decision, as well as investment considerations on the basis of component requirements.
If work pieces are subjected to extreme wear, yet at the same time must exhibit a high degree of dimensional stability, a satisfactory solution can often be found by using cemented carbides. An example of its use in the automobile construction sector are parts of the valve gear. Further applications include cutting edges for a wide variety of metal cutting tools.
Brazing has become by far the most important method of joining cemented carbides to supporting bases. Because of their predominantly carbide structure, cemented carbides, are only suitable for brazing under certain conditions. This disadvantage was compensated for by the development of carbide brazing alloys containing alloy components which promote wetting. The alloys used are silver- or copper-based, with zinc, nickel and/or manganese additives.
In addition to the various sorts of cemented carbides, highly abrasive materials and cutting materials such as ceramics, composite diamond materials or polycrystalline diamond (PKD) are also used for machining purposes. With these relatively new cutting materials, even hard stone can be cut very successfully. They are not only used in drilling wells, but also for quarrying and additional cutting of stone blocks. Here too, brazing is the dominant joining process for the deposition of hard material coatings.
Further methods of depositing wear-resistant layers involve brazing tungsten carbide of various particle sizes to a substrate. This enables not only tools for processing extremely hard and brittle materials (e.g. glass drills) to be manufactured, but also abrasive tools of the rubber industry.
Brazing alloys can be used not only to form wear-protecting bonds between the coating and substrate, but also possess outstanding wear-protecting characteristics of their own. Nickel-based brazing alloys for example have been derived from nickel hardfacing alloys of the NICrBSi system.
Flexible materials in the form of strips, mats or bands made of plastics that contain hard substances and a solder powder mix have been used for quite sometime to coat metallic bases with coatings made of a hard wear resistent substance. The production and design of these materials is described, for example, in DE-PS 38 01 958. These materials are also used for coating large surfaces (larger than 60.times.60 mm.sup.2) with thicknesses of between 0.5 to 6 mm. Typically in the industry the thickness of the flexible bands is 1 to 3 mm. The proportion of hard substance or brazing powder in the plastic materials is somewhere between 30 to 80 percent of volume. These flexible bands are so pliable that they can be laid up onto the surface of the work piece to be brazed so that it forms a skin. After heat treatment the layer is securely thermally bonded to the underlying work piece. One characteristic of all these materials is that this plastic binder burns out, volatilizing almost completely at temperatures of between 200.degree. C. to 600.degree. C., depending upon the plastic used. Such thermoplastic binders are well known in the art. A mound of braze metal remains that displays a mechanical interlocking with the hard substance, depending upon the percentage of hard substance and type of production. The result is a bonded coating between the work piece or working surface and the protective layer.
Once heated to temperatures above the liquidus temperature of the brazing powder mix, there results a stable compound. As a result, one obtains a protective coating formed of the brazing layer that contains the hard substance in finely distributed form.
In the coating of metallic bases with thermally volatilizable plastic materials that contain more than 40% in volume of hard substance and brazing powder, there appears relative motion between the protective coating layer and the underlying metallic body during heating at processing temperatures above 950.degree. C. The coating, which is usually designed for relatively large surfaces, is placed under such tensile stress that the coating, in its entirety, cannot withstand it. The consequence of these relative movements is uncontrolled crack formations with crack widths of up to 2 mm depending on the geometry of the coated surface. Cracks with a width of more than 0.5 mm are not completely filled by the solder alloy because of the lack of capillary filling pressure. Along with the crack effect that results because of geometric reasons, locally distinct differences appear in the mechanical characteristics that are caused in the cracks by the lack of hard substance in the compound or the coating and this produces errors that are not acceptable for parts that must have close tolerances on fit.